Where does “*-airu” come from?

*-airu (Vulgar Latin) comes from Latin -ārius, from Proto-Italic *-āzios.

Ancestry of “*-airu”, step by step

Step	Language	Word	Meaning
1	Latin	-ārius	—
2	Proto-Italic	*-āzios	—

*-airu — Etymology, Origin & Word History | Etymology Explorer